1. Field of Invention
The present disclosure relates to interferometers and, more particularly, to an optical heterodyne Michelson interferometer that employs active frequency feedback in order to measure vibrations over a large dynamic range with high sensitivity and minimal post-processing.
2. Description of Related Art
Mechanical vibration measurements may be obtained using a number of optical techniques. Such optical techniques may include, but are not limited to, laser speckle analysis, position-sensitive detector use, and holography. While execution of these optical techniques may vary considerably, many such optical techniques employ Michelson interferometers and thus rely on basic principles such as the Doppler effect. In accordance with the Doppler effect and its use in optical interferometric vibration measurements, light reflected from a moving surface experiences a frequency shift that is proportional to its instantaneous velocity.
For purposes of mechanical vibration measurements, data may be extracted from a Doppler-shifted beam. To determine the displacement of a mechanical vibration, the Doppler-shifted beam may be optically beaten with a reference beam. The phase of the beat signal is directly related to the displacement of the vibrating test object. To determine the velocity of a mechanical vibration, changes in the frequency of the beat signal may be measured.
Laser Doppler vibrometers (LDVs)—which often incorporate Michelson-based interferometers—may be employed in mechanical vibration measurements. Using these LDVs, it is customary to infer either the target displacement or target velocity from the interference between the reflected light and a reference beam.
Displacement measurements may be more desirable than velocity measurements since greater accuracy may be obtained through displacement measurements. However, it may be difficult to measure small displacements at high frequencies in the presence of large displacements at low frequencies.
There is a need for an optical technique for mechanical vibration measurements that nulls out the effect of large vibration excursions at low frequencies.
A number of prior art systems require phase unwrapping in order to obtain a vibration measurement. The processing associated with unwrapping the time-domain signal may impose harsh restrictions on data storage and the dynamic range on which the phase is digitized.
There is a need for an optical technique for mechanical vibration measurements that does not require such harsh restrictions on data storage and the dynamic range on which the phase is digitized.